what is c-web ?

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ICS-FORTH & Univ. of Crete Paris January 2000

Community Webs (C-Web): Functionality and

Architecture Issues

V. Christophides

Computer Science Department, University of CreteInstitute for Computer Science - FORTH

Heraklion, Crete

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What is C-Web ?

Set-up methodologies and infrastructure for fast deployment and easy management of knowledge-intensive Web applications in communities requiring:

effective knowledge assimilation, elicitation, ... efficient query answering

C-Web

education

healthcommerce

workplace

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C-Web Main Idea: Virtual XML Warehouse

Main Goal: to provide a generic platform for describing, organizing and querying various XML resources according to a concept taxonomy shared by a specific community

Single Point

of AccessFiles

Virtual XML Warehouse

Documents

Databases

Web

Knowledge

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C-Web Objectives

Reuse existing knowledge structures (e.g. ontologies, thesauri)

Integrate easily heterogeneous XML resources (e.g. data,

documents)

Provide an intelligent information access (i.e. conceptual

querying and browsing)

Support collaboration facilities & expertise management (e.g.

annotations)

Enable automatic generation of new information resources (e.g.

e-books)

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C-Web Functionality: Current Status

Support for creating community conceptual models integration of existing ontologies and thesauri definition different viewpoints

Support for describing and integrating resources resource content description metadata (CDM) resource structure mapping metadata (SMM)

Support for conceptual browsing and querying High level property-centric queries to resources Querying both conceptual schemata and related instances

Support for collaborative resource annotation Support for intelligent information publishing

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The main C-Web Requirement: Interoperability

Heterogeneity is not a drawback, but a feature of autonomous information resources in large scale distributed systems

Interoperability: the ability to uniformly share, interpret and manipulate data and documents from heterogeneous resources

domain & data models

vocabularies

communicationprotocols

transaction processing

security policies

data formats

abstraction &aggregation

details

Se

ma

ntic

Str

uctu

re

FunctionalSystem

Synta

ctic

viewpointscontexts

query language dialects

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Sculptor

Artist Artifact Museum

Painter Sculpture Painting Fine-Art Archeological

Impressionism

Neo-ImpressionismPointillism

BTCo

nce

ptu

al

Log

ica

lP

hys

ica

l

C-Web Design Principle: Repository Independence

Source 1: XML enabled DBMS

XML SQLXSQL Servlet

Source 2: XML Repository

<!ELEMENT MusArtifact (Name, Event+)><!ATTLIST MusArtifact material CDATA #IMPLIED size CDATA#IMPLIED><!ELEMENT Event (Person+,Place,Date)><!ATTLIST Event nature (creation|acquisition)><!ELEMENT Person (Name, Nation, Life?> ...

XQLXpathServlet

Domain Model

<elementType name=”ArtWork"> <sequence> <elementTypeRef name="Title" minOccur="1"/> <elementTypeRef name=”Creator" minOccur="1"/> </sequence>….</elementType>

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C-Web & Related W3C W3C Standards

Semantic Interoperability: Content Description & Metadata Standards ontologies (e.g. ICOM/CIDOC), thesauri (e.g., ULAN, TGN, AAT),

metadata element sets (e.g. CIMI/Aquarelle Z39.50 profile) Resource Description Framework (RDF) for expressing semantics

Structural Interoperability: Schema languages for specifying logical structure of Web resources

DTDs, XML Schema Syntactic Interoperability: Markup languages for exchanging (semi-)

structured data over the Web XML, XLL, ...

Functional Interoperability: Data Manipulation languages for (semi-) structured data over the Web

XPath, XQL, XSL, ...

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From RDF Schemata to XML resourcesR

DF

sch

ema

RD

F/X

ML

met

adat

aX

ML

Res

ourc

es

artist:Artist

artist:Painter artist:Sculptor

s

about

#August_Rodin

artist:lives_in

d

rdfs:Literal

Paris

artist:Sculptured

r

artist:sculpts

r

#The Burghers of Calais

#The Gates of Hell

s

s : rdfs:subclassOf

t : rdf:type

t tt

d: rdfs:domain r: rdfs:range

artist:lives_in

artist::sculpts

artist:sculpts

artist:materialartist:Material

Ironartist::material

rd

www.artist.gr/august_rodin

<ARTIST> <NAME>August Rodin <LIVES>Paris <WORK> <TITLE>The Gates of Hell <MATERIAL> Iron ……..

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XML Resources and C-Web Metadata

Artist

Name Lives Work

Title MaterialStyle

Museum Object

Natural Object

Artifactisa

Periodof period

has_style

Style

String

Materialconsists of

title

C-Web Schema XML Structure


<rdf:RDF xmlns:rdf="...#” xmlns:rdfs="...#" xmlns:s=”mycweb.forth.gr/...#"><rdf:Description about=”www.artist.gr"> <s:mappings> <rdf:Bag> <li><Description about=”Artist.Work.Title”> <s:map rdf:resource=“s:#title”/></li> <li><Description about=”Artist.Work.Material”> <s:map rdf:resource=“s:#Material”/></li> …</rdf:Bag></s:mappings>

C-Web

Resource

Description

Interface



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The C-Web Architecture

Query Engine

Session Manager

URL ResolverMetadataStore

Painter

Museum

Painting

XML Wrapper

ResourcesXMLXML

Logical Middle Tier

CWEB/APP Server

Other docs

news, reports

on the Intranete.g. mails,

Well-formedXML docson the Web

XML

XML enabledDBMS

Client Tier

XML/XSLRDF/XML

SchemaEditor

Virtual DocumentRender

RDF/XMLLoader

XML/XSLProcessor

Artist

URL

ResourceDescription

Interface

Query BrowsingInterface

ResourceAnnotationInterface

Artist

RDF/XML

Middleware APIs

http

http

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C-Web Middleware: Main Features

Genericity: capture any XML structure (various DTDs), any form of XML

semantics (DTDs, XML Schema), any XML access interface/protocol

(XQL, XLL)

Scalability: w.r.t. the volume of XML resources, the number of XML

repositories, network and server load, etc.

Extensibility: evolution of XML resources semantics and structure does

not affect the main processing components and interfaces

Openness: rely on standards & APIs allowing to plug and play the same

components & services in various applications, domains, etc.

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The C-Web Architecture: Pending Issues

Schema Editor: Standalone application or client of the Middleware? Where are stored large thesauri/ontology before their integration? Who is responsible for Schema Validation (from scratch vs. integrated)? What communication protocol we need with the C-Web Middleware?

Resource Description Interface: Loose or Tight coupled with the Middleware? How C-Web Schema browsing/querying is implemented? Where we can find the XML DTDs/Schemata of resources? Who is responsible for Resource Description Validation? What communication protocol we need with the C-Web Middleware?

Metadata Store: What persistence support we need? What is an efficient RDF storage model (indexing & clustering)? Did we also need to support updates/versions (versioning model)? What are the authentication/security policies (RDF/XML with signatures)? What is the result form of a C-Web query (triples, statements or objects)?

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C-Web Communication Protocol: 3 Alternatives

Client Server

Client Server

Client Server

Query

Reply

QueryHandle

NextNext

ReplyReply

ReplyReplyReply

Subscribe

Synchronous: a blocking query waits for an expected reply

Asynchronous: a nonblocking subscribe results in replies

Server maintains state; replies sent individually when requested

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Towards a C-Web Physical Architecture

MetadataStore

CWEB/APP Server

Schema Editor Resource DescriptionInterface

RDF/XMLParser Loader

MetadataStore

MetadataStore

RDF/XMLQuery Engine

Query BrowsingInterface

Session Manager

NM1

Ontologies

NM2

Thesauri

NM3C-Web

Schema &Instances

Internet

Ethernet

RDF/XMLSchema

RDF/XMLDescriptionsRDF-QL

PersistentNamespace

Service